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Abstract:

A near-eye display projects virtual images from an image generator to an
eyebox within which the virtual images can be seen by a viewer. A first
optical path conveys image-bearing light from the image generator to a
selectively reflective powered optic and a second optical path conveys
the image-bearing light along a line of sight from the selectively
reflective powered optic to the eyebox. First and second selectively
reflective surfaces fold the first optical path with respect to the
second optical path to locate the image generator out of the line of
sight to the eyebox. The image generator is effectively inclined to the
line of sight to the eyebox for reducing a thickness of the near-eye
display. The selectively reflective powered optic is oriented normal to
local overlapping portions of the first and second optical paths at the
selectively reflective powered optic.

Claims:

1. A near-eye display for projecting virtual images from an image
generator to an eyebox within which the virtual images can be seen by a
viewer comprising a selectively reflective powered optic connecting first
and second optical paths, the first optical path providing for conveying
image-bearing light from the image generator to the selectively
reflective powered optic and the second optical path providing for
conveying the image-bearing light along a line of sight from the
selectively reflective powered optic to the eyebox, first and second
selectively reflective surfaces folding the first optical path with
respect to the second optical path to locate the image generator out of
the line of sight to the eyebox, the image generator being effectively
inclined to the line of sight to the eyebox for reducing a thickness of
the near-eye display, and the selectively reflective powered optic being
oriented normal to local overlapping portions of the first and second
optical paths at the selectively reflective powered optic.

2. The near-eye display of claim 1 in which the image generator is
oriented normal to a local portion of the first optical path at the image
generator.

3. The near-eye display of claim 2 in which the first selectively
reflective surface and the selectively reflective powered optic have
optical axes oriented substantially parallel to the line of sight along
the second optical path and the second selectively reflective surface has
an optical axis oriented with respect to the axis of the first
selectively reflective surface through an angle of less than 45 degrees.

4. The near-eye display of claim 3 in which the second selectively
reflective surface is oriented with respect to the first selectively
reflective surface through an angle between 25 degrees and 35 degrees.

5. The near-eye display of claim 1 in which the image generator or a
relayed image of the image generator is inclined to the line of sight to
the eyebox.

6. The near-eye display of claim 1 in which the first selectively
reflective surface conveys the image-bearing light by at least one of
reflection and transmission at different points along the first optical
path from the image generator to the selectively reflective powered optic
and conveys image-bearing light by transmission along the second optical
path from the selectively reflective powered optic to the eyebox.

7. The near-eye display of claim 6 in which the second selectively
reflective surface that conveys the image-bearing light by reflection
along the first optical path from the image generator to the selectively
reflective powered optic and by transmission along the second optical
path from the selectively reflective powered optic to the eyebox.

8. The near-eye display of claim 1 in which the selectively reflective
powered optic is a holographic optic arranged for focusing the
image-bearing light and transmitting ambient light along the line of
sight to the eyebox.

9. The near-eye display of claim 8 in which the second selectively
reflective surface includes a polarization-sensitive beamsplitter and a
polarization modifier is located between the first selectively reflective
surface and the selectively reflective powered optic.

10. The near-eye display of claim 9 in which the first and second
selectively reflective surfaces are unpowered optical surfaces.

11. A near-eye display for projecting virtual images from an image
generator to an eyebox within which the virtual images can be seen by a
viewer comprising a selectively reflective powered optic connecting first
and second effectively parallel optical paths, the first optical path
providing for conveying image-bearing light from the image generator to
the selectively reflective powered optic and the second optical path
providing for conveying the image-bearing light from the selectively
reflective powered optic to the eyebox, first and second selectively
reflective surfaces each encountering the image-bearing light along the
first and second optical paths, the first selectively reflective surface
providing for conveying the image-bearing light from the image generator
to the second selectively reflective optic and from the second
selectively reflective optic to the selectively reflective powered optic
along the first optical path and providing for conveying the
image-bearing light from the selectively reflective powered optic to the
second selectively reflective surface along the second optical path, and
the second selectively reflective surface providing for conveying the
image-bearing light from the first selectively reflective surface back to
the first selectively reflective surface along the first optical path and
providing for conveying the image-bearing light from the first
selectively reflective surface to the eyebox along the second optical
path.

12. The near-eye display of claim 11 in which the selectively reflective
powered optic and the first selectively reflective surface have optical
axes oriented substantially parallel to the second optical path.

13. The near-eye display of claim 12 in which the second selectively
reflective surface also has an optical axis and the optical axes of the
first and second selectively reflective surfaces are relatively inclined
by an angle of less than 45 degrees.

14. The near-eye display of claim 13 in which the second selectively
reflective surface is oriented with respect to the first selectively
reflective surface through an angle between 25 degrees and 35 degrees.

15. The near-eye display of claim 11 in which the first and second
selectively reflective surfaces are supported by facets of a prismatic
waveguide that also includes an entrance facet along the first optical
path from the image generator oriented substantially normal to the first
optical path at the entrance facet.

16. The near-eye display of claim 15 in which the first selectively
reflective surface reflects the image-bearing light within the prismatic
waveguide by a mechanism of total internal reflection.

17. The near-eye display of claim 15 further comprising a supplemental
prism having an adjoining facet adjacent to the second selectively
reflective surface and an exit facet oriented parallel to the first
selectively reflective surface.

18. The near-eye display of claim 17 in which the selectively reflective
powered optic, the first selectively reflective surface supported by the
prismatic waveguide, and the exit facet of the supplemental prism all
have optical axes oriented parallel to the second optical path to the
eyebox.

19. The near-eye display of claim 18 in which a see-through optical path
is located in parallel with the second optical path for conveying ambient
light through the selectively reflective powered optic, the first and
second selectively reflective surfaces supported by the prismatic
waveguide, and through the entrance and exit facets of the supplemental
prism to the eyebox.

20. The near-eye display of claim 11 in which the selectively reflective
powered optic is a reflective powered hologram.

21. The near-eye display of claim 20 in which the first selectively
reflective surface is a reflective unpowered hologram.

22. The near-eye display of claim 11 in which a see-through optical path
is located in parallel with the second optical path for conveying ambient
light through the selectively reflective powered optic and the first and
second selectively reflective surfaces to the eyebox.

23. The near-eye display of claim 11 in which the first selectively
reflective surface provides for conveying the image-bearing light (a) by
reflection along the first optical path from the image generator to the
second selectively reflective surface, (b) by transmission along the
first optical path from the second selectively reflective surface to the
selectively reflective powered optic, and (c) by transmission along the
second optical path from the selectively reflective powered optic to the
second selectively reflective surface.

24. The near-eye display of claim 23 in which the second selectively
reflective surface provides for conveying the image-bearing light (a) by
reflection along the first optical path from the first selectively
reflective surface back to the first selectively reflective surface and
(b) by transmission along the second optical path from the first
selectively reflective surface to the eyebox.

25. A compound display and imaging device for displaying virtual images
from within an eyebox and for imaging external views from a perspective
of the eyebox comprising a camera, a selectively reflective powered
optic, and first and second selectively reflective surfaces, the
selectively reflective powered optic providing for projecting virtual
images from an image generator along a display optical pathway to an
eyebox having a field of view within which the virtual images are
visible, the first and second selectively reflective surfaces each
providing for conveying display light along the display pathway from the
image generator to the selectively reflective powered optic and from the
selectively reflective powered optic to the eyebox, the camera providing
for imaging the external views along an image pathway through the
selectively reflective powered optic, the first and second selectively
reflective surfaces each providing for conveying image light along the
image pathway from the selectively reflective powered optic to the
camera, and the image pathway between the second selectively reflective
optic and the selectively reflective powered optic being aligned with the
display pathway between the selectively reflective powered optic and the
eyebox so that the external views imaged by the camera are aligned with
the virtual images that are visible from within the eyebox.

26. The compound display and imaging device of claim 25 in which a
see-through pathway is aligned with both (a) the image pathway between
the second selectively reflective optic and the selectively reflective
powered optic and (b) the display pathway between the selectively
reflective powered optic and the eyebox so that external views apparent
from within the eyebox correspond to the external views that are imaged
by the camera.

27. The compound display and imaging device of claim 25 in which the
first selectively reflective optic conveys display light along the
display pathway to the second selectively reflective optic by one of
reflection or transmission and conveys image light along the image
pathway to the second selectively reflective optic by the other of the
reflection or transmission.

28. The compound display and imaging device of claim 27 in which the
second selectively reflective optic conveys both (a) the display light
along the display pathway to the selectively reflective powered optic and
(b) the image light along the image pathway from the selectively
reflective powered optic.

29. The compound display and imaging device of claim 25 further
comprising a prismatic waveguide assembly including a display entrance
facet along the display pathway and an image entrance facet along the
image pathway.

30. The compound display and imaging device of claim 29 in which the
first and second selectively reflective surfaces are supported on facets
of the prismatic waveguide assembly that are relatively inclined through
an angle of less than 45 degrees.

31. The compound display and imaging device of claim 30 in which the
prismatic waveguide assembly includes an inner supplemental prism having
an adjoining facet adjacent to the second selectively reflective surface
and an inner face facet oriented parallel to the first selectively
reflective surface.

32. The compound display and imaging device of claim 31 in which the
prismatic waveguide assembly includes an outer supplemental prism having
an adjoining facet adjacent to the selectively reflective powered optic
and an outer face facet oriented parallel to the inner face facet.

33. The compound display and imaging device of claim 25 in which the
second selectively reflective surface is a polarization-sensitive
beamsplitter that reflects one orientation of polarized light and
transmits another orientation of polarized light.

34. The compound display and imaging device of claim 33 in which the
polarization-sensitive beamsplitter reflects the image light from the
external view toward the camera, reflects the display light from the
image generator toward the selectively reflective powered optic, and
transmits the display light from the selectively reflective powered optic
toward the eyebox.

35. The compound display and imaging device of claim 34 further
comprising a polarization modifier for changing the orientation of the
polarized light located between the first selectively reflective surface
and the selectively reflective powered optic.

36. The compound display and imaging device of claim 25 in which the
camera includes a focusing optic for focusing the external views within
the camera, and the selectively reflective powered optic transmits the
image light along the image pathway to the camera without contributing
focusing power.

37. A near-eye augmented reality device comprising an image generator, a
camera, and a selectively reflective powered optic, the selectively
reflective powered optic providing for projecting virtual images from the
image generator along a display optical pathway to an eyebox having a
field of view within which the virtual images are visible, the camera
providing for imaging external views along an image pathway through the
selectively reflective powered optic, a see-through pathway extending
through the selectively reflective powered optic to the eyebox for
transmitting the external views to the eyebox, a portion of the image
pathway through the selectively reflective powered optic being aligned
with a portion of the display pathway between the selectively reflective
powered optic and the eyebox so that the external views imaged by the
camera are aligned with the virtual images that are visible from within
the eyebox, and a portion of the image pathway through the selectively
reflective powered optic being aligned with the see-through pathway to
the eyebox so that the external views apparent from within the eyebox
correspond to the external views that are imaged by the camera.

38. The device of claim 37 further comprising a first selectively
reflective surface that reflects light along one of the image pathway and
the display pathway and transmits light along the other of the image
pathway and the display pathway for directing light to the camera along
the image pathway and directing light from the image generator along the
display pathway.

39. The device of claim 38 further comprising a second selectively
reflective surface that reflects light along one of the image pathway and
the see-through pathway and transmits light along the other of the image
pathway and the see-through pathway for directing light to the camera
along the image pathway and directing light to the eyebox along the
see-through pathway.

40. The device of claim 39 in which light from the image generator twice
encounters the first selectively reflective optic en route to the eyebox
and light from the external views twice encounters the first selectively
reflective optic en route to the camera.

[0002] Near-eye displays, particularly those that also provide see-through
views of the ambient environment, incorporate powered optics for forming
virtual images without unduly obstructing views of the ambient
environment. Some such displays fold light paths to the powered optics
out of the line of sight to the ambient environment and in others, the
powered optics are at least partially transparent to light from the
ambient environment.

[0003] The powered optics folded out of the line of sight generally add to
another dimension of the displays, particularly to the thickness of the
displays. Such additional thickness is often undesirable. Near-eye
displays are generally formed as thin as possible to more closely
replicate the styles of other eyewear.

[0004] The powered optics that are located along the line of sight are at
least partially transmissive to ambient light but are generally oriented
off axis to improve overall light efficiency and to reduce thickness
requirements of other display optics for directing image-bearing light to
the powered optics. The focusing power of the powered optics is expressed
under reflection. To preserve a natural view of the ambient environment,
either an additional optic is required to undo the focusing power under
transmission or the powered optic is formed as a holographic optical
element that transmits ambient light largely undisturbed. The off-axis
orientation of the in-line powered optics often requires corrections for
both image distortion and chromatic aberration.

SUMMARY OF THE INVENTION

[0005] The invention among it preferred embodiments, features a near-eye
display with a selectively reflective powered optic (also meant to be
selectively transmissive as well) oriented nominally normal to a line of
sight to the ambient environment. Optical paths to and from the
selectively reflective powered optic effectively overlap. That is,
although the optical path to the selectively reflective powered optic is
folded to locate an image generator out of the line of sight, the image
generator and the selectively reflective powered optic both remain
nominally normal to local portions of the optical path between them. The
common optical alignment between the image generator and the selectively
reflective powered optic reduces issues of image distortion and chromatic
aberration, largely by maintaining rotational symmetry.

[0006] In addition, the folded light path to the selectively reflective
powered optic allows the image generator or its relayed image to be
inclined (i.e., effectively inclined) to a thickness direction of the
near-eye display along the line of sight. The inclined orientation of the
image generator or its relayed image enables the construction of a
thinner, i.e., more compact, near-eye display.

[0007] One version of a near-eye display in accordance with the invention
projects virtual images from an image generator to an eyebox within which
the virtual images can be seen by a viewer. A selectively reflective
powered optic connects first and second optical paths. The first optical
path conveys image-bearing light from the image generator to the
selectively reflective powered optic, and the second optical path conveys
the image-bearing light along a line of sight from the selectively
reflective powered optic to the eyebox. First and second selectively
reflective surfaces fold the first optical path with respect to the
second optical path to locate the image generator out of the line of
sight to the eyebox. The image generator is effectively inclined to the
line of sight to the eyebox for reducing a thickness of the near-eye
display, and the selectively reflective powered optic is oriented
nominally normal to local overlapping portions of the first and second
optical paths at the selectively reflective powered optic for reducing
image distortion.

[0008] Preferably, the image generator is also oriented nominally normal
to a local portion of the first optical path at the image generator for
reducing image distortion. The first selectively reflective surface and
the selectively reflective powered optic have optical axes preferably
oriented substantially parallel to the line of sight along the second
optical path. The second selectively reflective surface has an optical
axis preferably oriented with respect to the axis of the first
selectively reflective surface through an angle of less than 45 degrees
and more preferably between 25 degrees and 35 degrees.

[0009] The selectively reflective powered optic is preferably a
holographic optic arranged for focusing the image-bearing light within
the eyebox and transmitting ambient light along the line of sight to the
eyebox. The second selectively reflective surface preferably includes a
polarization-sensitive beamsplitter, and a polarization modifier is
preferably located between the first selectively reflective surface and
the selectively reflective powered optic. The first and second
selectively reflective surfaces are preferably unpowered optical
surfaces.

[0010] Another version of a near-eye display for projecting virtual images
from an image generator to an eyebox within which the virtual images can
be seen by a viewer includes a selectively reflective powered optic
connecting first and second effectively parallel optical paths (i.e.,
paths that would be parallel if unfolded from reflection). The first
optical path conveys image-bearing light from the image generator to the
selectively reflective powered optic, and the second optical path conveys
the image-bearing light from the selectively reflective powered optic to
the eyebox. First and second selectively reflective surfaces each
encounter the image-bearing light along the first and second optical
paths. The first selectively reflective surface conveys the image-bearing
light from the image generator to the second selectively reflective optic
and from the second selectively reflective optic to the selectively
reflective powered optic along the first optical path. In addition, the
first selectively reflective surface conveys the image-bearing light from
the selectively reflective powered optic to the second selectively
reflective surface along the second optical path. The second selectively
reflective surface conveys the image-bearing light from the first
selectively reflective surface back to the first selectively reflective
surface along the first optical path and conveys the image-bearing light
from the first selectively reflective surface to the eyebox along the
second optical path.

[0011] Preferably, facets of a prismatic waveguide support the first and
second selectively reflective surfaces. An entrance facet of the
prismatic waveguide is preferably oriented nominally normal to the first
optical path at the entrance facet. The first selectively reflective
surface preferably reflects the image-bearing light within the prismatic
waveguide by a mechanism of total internal reflection.

[0012] For viewing the ambient environment along the second optical path,
a supplemental prism can be provided with an adjoining facet adjacent to
the second selectively reflective surface and an inner face facet
oriented parallel to the first selectively reflective surface.
Preferably, the selectively reflective powered optic, the first
selectively reflective surface supported by the prismatic waveguide, and
the inner face facet of the supplemental prism all have optical axes
oriented parallel to the second optical path to the eyebox.

[0013] A version of a compound display and imaging device in accordance
with the invention for both displaying virtual images from within an
eyebox and for imaging external views from a perspective of the eyebox
includes a camera, a selectively reflective powered optic, and first and
second selectively reflective surfaces. The selectively reflective
powered optic projects virtual images from an image generator along a
display optical pathway to an eyebox having a field of view within which
the virtual images are visible. The first and second selectively
reflective surfaces each convey display light along the display pathway
from the image generator to the selectively reflective powered optic and
from the selectively reflective powered optic to the eyebox. The camera
images the external views along an image pathway through the selectively
reflective powered optic. The first and second selectively reflective
surfaces each convey image light along the image pathway from the
selectively reflective powered optic to the camera. The image pathway
between the second selectively reflective optic and the selectively
reflective powered optic is aligned with the display pathway between the
selectively reflective powered optic and the eyebox so that the external
views imaged by the camera are aligned with the virtual images that are
visible from within the eyebox.

[0014] Preferably, a see-through pathway is aligned with both (a) the
image pathway between the second selectively reflective optic and the
selectively reflective powered optic and (b) the display pathway between
the selectively reflective powered optic and the eyebox so that external
views apparent from within the eyebox correspond to the external views
that are imaged by the camera. The first selectively reflective optic
preferably conveys display light along the display pathway to the second
selectively reflective optic by one of reflection or transmission and
preferably conveys image light along the image pathway to the second
selectively reflective optic by the other of the reflection or
transmission. The second selectively reflective optic preferably conveys
both (a) the display light along the display pathway to the selectively
reflective powered optic and (b) the image light along the image pathway
from the selectively reflective powered optic.

[0015] A prismatic waveguide assembly within the compound display and
imaging device preferably includes a display entrance facet along the
display pathway and an image entrance facet along the image pathway. The
first and second selectively reflective surfaces are preferably supported
on facets of the prismatic waveguide assembly that are relatively
inclined through an angle of less than 45 degrees and more preferably
between 25 degrees and 35 degrees. An inner supplemental prism of the
prismatic waveguide assembly preferably includes an adjoining facet
adjacent to the second selectively reflective surface and an inner face
facet oriented parallel to the first selectively reflective surface. An
outer supplemental prism of the prismatic waveguide assembly preferably
includes an adjoining facet adjacent to the selectively reflective
powered optic and an outer face facet oriented parallel to the inner face
facet.

[0016] The second selectively reflective surface is preferably a
polarization-sensitive beamsplitter that reflects one orientation of
polarized light and transmits another orientation of polarized light. For
example, the polarization-sensitive beamsplitter can be arranged to
reflect the image light from the external view toward the camera, reflect
the display light from the image generator toward the selectively
reflective powered optic, and transmit the display light from the
selectively reflective powered optic toward the eyebox. The camera
preferably includes a focusing optic for focusing the external views
within the camera, and the selectively reflective powered optic
preferably transmits the image light along the image pathway to the
camera without contributing focusing power.

[0017] A version of near-eye augmented reality device in accordance with
the invention includes both a camera and a selectively reflective powered
optic. The selectively reflective powered optic projects virtual images
from an image generator along a display optical pathway to an eyebox
having a field of view within which the virtual images are visible. The
camera images external views along an image pathway through the
selectively reflective powered optic. A see-through pathway extends
through the selectively reflective powered optic to the eyebox for
transmitting the external views to the eyebox. A portion of the image
pathway through the selectively reflective powered optic is aligned with
a portion of the display pathway between the selectively reflective
powered optic and the eyebox so that the external views imaged by the
camera are aligned with the virtual images that are visible from within
the eyebox. A portion of the image pathway through the selectively
reflective powered optic is aligned with the see-through pathway to the
eyebox so that the external views apparent from within the eyebox
correspond to the external views that are imaged by the camera.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0018] FIG. 1 is a diagrammatic top view of a near-eye display in
accordance with the invention.

[0019] FIG. 2 is a diagrammatic top view of a compound display and imaging
device 60 in accordance with the invention.

[0020] FIG. 3 is a similar diagrammatic top view of the display of FIG. 1
simplified for showing orientations and relationships with respect to
paraxial rays propagating through the display.

DETAILED DESCRIPTION OF THE INVENTION

[0021] A near-eye display 10, as depicted in FIG. 1, includes an image
generator 12 for producing a succession of images, such as video images,
that are projected as virtual images into an eyebox 14. The image
generator 12, which can take a number of forms, preferably combines a
spatial light modulator 16 with an illuminator 18 that uniformly
illuminates the spatial light modulator 16. The illuminator 18 preferably
includes a light source 17, which can be formed by one or more light
emitting diodes or other known sources including lamps, and a condenser
19 that collects light from the source 17 and evenly illuminates the
spatial light modulator 16. Light patterns are produced within the
spatial light modulator 16 by differentially propagating light on a
pixel-by-pixel basis in accordance with a video input signal from a video
source (not shown). For example, the spatial light modulator 16 can
comprise a controllable array of liquid crystal diodes functioning as
individually addressable pixels for producing desired light patterns in
response to the video signal. Other spatial light modulators useful for
purposes of the invention include grating light valve (GLV) technologies
and digital light processing (DLP) technologies such as digital
micromirror devices (DMDs). The spatial light modulator 16 and
illuminator 18 can be replaced by self-illuminating image generators in
which the addressable pixel elements are themselves individually
controllable light sources, such as organic light-emitting diode (OLED)
technologies.

[0022] Within the near-eye display 10, the output of the image generator
12 is effectively located within an object plane 20, which is intended to
be projected as a magnified virtual image within the eyebox 14. Light
emitted from the image generator 12 enters a prismatic waveguide 22
through an entrance facet 24 that is oriented in parallel with the object
plane 20 so that chief rays from the object plane 20 or at least the
paraxial rays encounter the entrance facet 24 at near normal incidence.

[0023] Light emitted from the image generator 12 propagates along a first
optical path 26 through the prismatic waveguide 22 to a selectively
reflective powered optic 30, which focuses the light from the image
generator 12 by reflection along a second effectively parallel optical
path 28 through the prismatic waveguide 22 to the eyebox 14. The focal
power of the selectively reflective powered optic 30 projects a magnified
virtual image of the object plane 20, which is viewable from within the
eyebox 14. The selectively reflective powered optic 30 is oriented
substantially normal to local portions of first and second optical paths
26 and 28, i.e., the optical axis 31 of the selectively reflective
powered optic 30 is aligned with the first and second optical paths 26
and 28 along their opposing directions of incidence and reflection at the
selectively reflective powered optic.

[0024] Preferably, the selectively reflective powered optic 30 is formed
as a reflective volume hologram arranged for focusing selected
wavelengths of light. The selected wavelengths preferably correspond to a
red-green-blue (RGB) color combination output by the image generator 12.
Image-bearing light in the selected colors is reflected and focused along
the second optical path 28. The remaining wavelengths of visible light,
which include other wavelengths red, green, and blue light beyond the
coherent bands over which the volume hologram is formed, transmit through
the volume hologram largely undisturbed.

[0025] Before reaching the selectively reflective powered optic 30 along
the first optical path 26, first and second selectively reflective
surfaces 34 and 36 fold image-bearing light from the image generator 12
out of physical alignment with the eyebox 14. Except for the two
reflections at the selectively reflective surfaces 34 and 36, which do
not appreciably alter or otherwise distort the wavefront shape of the
image-bearing light, the first and second optical paths 26 and 28 remain
effectively optically aligned. In other words, if the first optical path
26 were to be physically unfolded, the first optical path from the image
generator 12 to the selectively reflective powered optic 30 would overlie
the second optical path 28 from the selectively reflective powered optic
30 to the eyebox 14. Since the object plane 20 lies substantially normal
to the first optical path 26 and the first and second optical paths 34
and 36 are effectively optically aligned with one another, the virtual
image is formed without distortion or any requirement for correcting
distortion along either of the two optical paths 26 and 28.

[0026] The first and second selectively reflective surfaces 34 and 36 are
supported by or otherwise formed by facets 42 and 44 of the prismatic
waveguide 22. The first selectively reflective surface 34 is formed on or
by the facet 42 and exhibits selective reflectivity by the mechanism of
total internal reflection (TIR), which exploits a difference between the
refractive index of the prismatic waveguide 22 and the refractive index
of the local environment adjacent to the facet 42. Coatings or other
modifications to the facet 42 can be used to influence the reflective
properties of the first selectively reflective surface 34. Preferably
light from the image generator 12 first encounters the first selectively
reflective surface 34 along the first optical path 26 at angles of
incidence greater than the critical angle above which the light is
totally internally reflected.

[0027] The second selectively reflective surface 36 is preferably
fashioned as a polarization-sensitive beamsplitter 40 supported on the
facet 44 of the prismatic waveguide 22. At least a portion of the
image-bearing light from the image generator 12 last reflected from the
first selectively reflective surface 34 is further reflected by the
second selectively reflective surface 36 along the first optical path 26
through the first selectively reflective surface 34 to the selectively
reflective powered optic 30. En route top the selectively reflective
powered optic 30 the image-bearing light encounters both the first
selectively reflective surface 34 and a polarization modifier 46 at a
nominal normal incidence. The near zero incidence angle at which the
image-bearing light encounters the first selectively reflective surface
34 is well below the critical angle required to support total internal
reflection within the prismatic waveguide 22. So, while the first
encounter of the image-bearing light with the first selectively
reflective surface 34 along the first optical path 26 is reflective, the
second encounter of the image-bearing light with the first selectively
reflective surface 34 is along the first optical path 26 is transmissive.

[0028] Instead of exploiting the mechanism of total internal reflection
(TIR) to support the function of selective reflectivity, the second
selectively reflective surface 36 in the form of the
polarization-sensitive beamsplitter 40 exploits the mechanism of
polarization to support the function of selective reflectivity. The light
emitted from the image generator 12 can be at least partially polarized,
especially if a liquid crystal array is used in combination with
orthogonal polarizers as the spatial light modulator 16. The second
selectively reflective surface 36 as a polarization-sensitive
beamsplitter is preferably arranged to reflect the polarization
orientation of the image-bearing light from the image generator 12 and to
transmit an orthogonal orientation of polarization.

[0029] The polarization modifier 46, which is spaced from the first
selectively reflective surface 34 to preserve an air gap 48 or other
low-reflectivity medium required for sustaining total internal
reflection (TIR), modifies the polarization of the image bearing light by
relatively retarding one orthogonal polarization component with respect
to another through approximately π/2 radians. After being both
reflected and focused by the selectively reflective powered optic 30, the
image-bearing light propagates along the second optical path 28 through
both the polarization modifier 46 and the first selectively reflective
surface 34 to the second selectively reflective surface 36. Since the
optical axis 31 of the selectively reflective powered optic 30 is aligned
with local portions of the first and second optical paths 26 and 28, the
focused image-bearing light reflected from the selectively reflective
powered optic 30 transmits through both the polarization modifier 46 and
the first selectively reflective surface 34 at near normal incidence and
well below the critical angle for TIR. The second encounter of the
image-bearing light with the polarization modifier 46 relatively modifies
the orthogonal polarization components of the image-bearing light by
another π/2 radians, so the total modification from both encounters
orthogonally transforms the polarization of the image-bearing light. A
one-quarter wave plate retarder is preferably used to perform the
two-step polarization modification. The focused image-bearing light
propagating along the second optical path 28 transmits through the second
selectively reflective surface 36 (i.e., the polarization-sensitive
beamsplitter 40) as orthogonally rotated polarized light en route to the
eyebox 14.

[0030] While the entrance facet 24 is substantially normal to the local
portion of the first optical path 26 to minimize chromatically sensitive
refractive effects, the facet 44, on which the second selectively
reflective surface 36 is formed as the polarization-sensitive
beamsplitter 40 and through which the second optical path 28 exits the
prismatic waveguide 22, is not substantially normal to the second optical
path 28. Instead, the second selectively reflective surface 36 is
inclined to the first selectively reflective surface 34 through an acute
angle that is preferably less than 45 degrees and more preferably between
25 degrees and 35 degrees. At 30 degrees, image bearing light entering
the prismatic waveguide 22 propagates parallel to the second selectively
reflective surface 36 (i.e., the facet 44) en route to the first
selectively reflective surface 34 (i.e., the facet 42) for efficiently
filling the prismatic waveguide 22.

[0031] A supplemental prism 50 having an adjoining facet 52 mated to the
polarization-sensitive beamsplitter on the selectively reflective surface
36 on the facet 44 of the prismatic waveguide 22 and having an exit facet
54 parallel to the selective reflective surface 32 on the facet 42 of the
prismatic waveguide minimizes chromatically sensitive refractive effects
on the image bearing light propagating toward the eyebox 14. The
minimized prismatic refraction also avoids a shift in the viewing
position of the projected virtual image within the eyebox 14. To this
end, the refractive index of the supplemental prism 50 preferably matches
the refractive index of the prismatic waveguide 22.

[0032] The mating prisms, i.e., the prismatic waveguide 22 and the
supplemental prism 50, function together as a single plane parallel plate
with respect to a see-through pathway 56 in alignment with the second
optical path 28. However, the supplemental prism 50 and the prismatic
waveguide 22 are preferably mated together in an offset position so that
their combined thickness as a plane parallel plate along the second
optical pathway 28 and the see-through pathway 56 is less than the sum of
their individual thicknesses. The offset between the prismatic waveguide
22 and the supplemental prism 50 allows the entrance facet 24 to be sized
independently of the combined thicknesses of the prismatic waveguide 22
and the supplemental prism 50 plate in front of a viewer's eye.

[0033] Ambient light enters the near eye display 10 through the
selectively reflective powered optic 30, preferably in the form of a
volume hologram, and propagates along the see-through pathway 56 through
the polarization modifier 46 and the mating prisms 22 and 50 and into the
eyebox 14 in alignment with the image-bearing light focused into the
eyebox by the selectively reflective powered optic 30. The
polarization-sensitive beamsplitter located between the mating prisms 22
and 50 reflects a portion of the ambient light, preferably so-called
"S-polarized" light, which is prone to reflections off ground oriented
surfaces. Thus, the near-eye display 10 functions similar to polarized
sunglasses with respect to ambient light while also projecting virtual
images from the image generator 12 that are visible within the eyebox 14.

[0034] A compound display and imaging device 60 for displaying virtual
images from within an eyebox 14 and for imaging external views from a
perspective of the eyebox 14 is disclosed in FIG. 2. Together with known
software within a processor 58 for processing information imaged from the
external views and for incorporating the imaged information in some form,
such as computer-generated or computer-modified imagery, into the
displayed virtual images, the device 60 can be used as a mediated reality
device. For example, the imaged information can be processed for purposes
of object recognition and information about the recognized objects can be
incorporated into the displayed virtual images. Information can also be
gathered from the imaged external views that would not normally be
visible to a viewer using the device 60 (e.g., infrared light),
processed, and incorporated into the displayed images for enhancing the
viewer's view or other sensory appreciation of the external environment.

[0035] A display portion of the device 60 is similar to the near-eye
display 10 of FIG. 1 except that the selectively reflective surface 34 is
rendered selectively reflective by a volume hologram 62 instead of by the
mechanism of total internal reflection (TIR). In addition, since no gap
is required to sustain TIR, the polarization modifier 46 and the
selectively reflective powered optic 30 (e.g., a powered volume hologram)
can be moved alongside the volume hologram 62. For sake of simplicity,
features of the compound display and imaging device 60 in common with the
near-eye display 10 of FIG. 1 share the same reference numerals.

[0036] Three optical pathways are defined through the device 60: a display
pathway 64, an image pathway 66, and a see-through pathway 68. The
display pathway 64 corresponds to the first and second optical paths 26
and 28 through the device 10 for projecting virtual images that are
visible within the eyebox 14. The image pathway 66 conveys to a camera 70
external views corresponding at least in part to an ambient view of the
along the see-through pathway 68 to the eyebox 14. The see-through
pathway 68 corresponds to the see-through pathway 56 of the display 10
but is extended to accommodate additional optics associated with the
image pathway 66.

[0037] In addition to the structures in common with the display 10, the
device 60 includes a prismatic waveguide 72 and a supplemental prism 74.
The prismatic waveguide 72 is primarily intended for diminishing
refractive effects along the imaging pathway 66 associated with ambient
light passing through the first selectively reflective surface 34 on the
facet 42 of the prismatic waveguide 22. The supplemental prism 74 is
primarily intended for diminishing refractive effects along the combined
imaging and see-through pathways 66 and 68 associated with ambient light
passing through a facet 76 of the prismatic waveguide 72. Alternatively,
the prismatic waveguide 72 and the supplemental prism 74 could be
combined into a single prismatic waveguide, particularly since the facet
76 of the prismatic waveguide 72 is not required to perform an
independent optical function in the depicted embodiment. An index
matching adhesive 78 joins the prismatic waveguide 72 and the
supplemental prism 74 to the prismatic waveguide 22 and the supplemental
prism 50.

[0038] Ambient light along both the imaging pathway 66 and the see-through
pathway 68 enters the supplemental prism 74 through an entrance facet 80
and passes through an interface between the facet 76 of the prismatic
waveguide 72 and an adjoining facet 82 of the supplemental prism 74. An
inner face facet 84 of the prismatic waveguide 72 adjoins the selectively
reflective powered optic 30 along at least part of its length and the
index matching adhesive 78 couples the remaining common portions of the
two prismatic waveguides 22 and 72 and the two supplemental waveguides 50
and 74.

[0039] Light exiting the prismatic waveguide 72 along the imaging pathway
66 enters the prismatic waveguide 22 through various interfaces including
through a combination of the selectively reflective powered optic 30,
particularly as a powered hologram, the polarization modifier 46, and the
selectively reflective surface 34, particularly as a angular sensitive
reflective hologram and through a combination of the index matching
adhesive 78 and the selectively reflective surface 34.

[0040] The selectively reflective surface 36, particularly as a
polarizations-sensitive beamsplitter 40, reflects a portion of the light
along the imaging pathway back through the same interfaces between the
prismatic waveguides 22 and 72 en route to the camera 70. Substantially
the entire remaining portion of the light is transmitted through the
polarization sensitive beamsplitter 40 along the see-through pathway 68
en route to the eyebox 14.

[0041] Along the imaging pathway 66, light exits the prismatic waveguide
72 through an exit facet 86 that is oriented normal to the imaging
pathway 66. The camera 70, which completes the imaging pathway 66,
includes a focusing optic 87 and a detector 89. The focusing optic 87
images views within the ambient environment onto the detector 89, which
is located at an imaging plane. The detector 89 is preferably a digital
image capturing device such as a charge coupled device (CCD) array. The
processor 58 receives image information captured by the camera 70.

[0042] Along the see-through pathway 68, ambient light exits the prismatic
waveguide 72 through the exit facet 54, which extends parallel with the
entrance facet 80. Thus, the device 60 functions largely as a
plane-parallel plate along the see-through pathway 68 for avoiding
prismatic or other refractive effects, at least with respect to the chief
rays (or at least paraxial rays of systems that depart more significantly
from telecentricity) transmitted through the device 60. The optical path
portion 28 along the display pathway 64 is aligned with and overlaps the
see-through pathway 68 within the prismatic waveguide 22 and is aligned
with and overlaps the imaging pathway within a portion of the prismatic
waveguide 22 between the first selectively reflective surface 34 and the
exit facet 54. If unfolded from reflection, all three pathways 64, 66,
and 68 would be aligned with each other along a common optical axis and
substantially centered within the eyebox 14.

[0043] The processor 58, which can have access to additional information
in memory or from other sources, such a global positioning system data,
processes the digital image information from the camera 70 taken largely
along the line of sight of the see-through pathway 68. The processed
information is preferably used to generate text or graphics that are
reproduced by the image generator 12 for projection as virtual images
that can be overlaid onto the viewer's view of the external world along
the same line of sight. The text and graphics can overlay or reference
particular features that are visible along the see-through pathway 68 or
other features that are not visible or are only marginally visible to a
viewer. To precisely associate projected virtual images of text or
graphics with particular features in the external world, the images
received the camera 70 should be optically aligned and scaled to the
images reproduced by the image generator 12 and both the images received
by the camera 70 and the images reproduced by the image generator 12
should be optically aligned and scaled to the images of the external
world that are visible to a viewer along the see-through pathway 68.

[0044] With reference to FIG. 3 depicting the propagation of paraxial rays
through the near-eye display 10 of FIG. 1 and assuming that (a) the first
selectively reflective surface 34 is oriented normal to the optical axis
31 of the selectively reflective powered optic 30 and (b) the second
selectively reflective surface 36 reflects light (i.e., at least the
paraxial rays) along the same optical axis 31, an incidence angle
"δ" at which the paraxial rays approach the first selectively
reflective surface 34 is equal to two times an angle "θ" at which
the second selectively reflective surface 36 is inclined to the first
selectively reflective surface 34. As also apparent from FIG. 3, the
object plane 20 of the image generator 12 is inclined (i.e., an effective
inclination of the image generator 12) with respect to the optical axis
31 of the selectively reflective focusing optic 30 and the see-through
pathway 68 through an angle equal to the complement of the incidence
angle "δ", which allows the display 10 to be narrower in the
direction of the see-through pathway 68. En route to the first
selectively reflective surface 34, the inclination of the object plane 20
can reduce the local thickness of the display by the Cosine function of
the inclination angle.

[0045] The illustrated designs support inclination angles "θ" of
less than 45 degrees for limiting the thickness of the devices 10 and 60
along the see-through pathway 68. Preferably, the inclination angles
"θ" approach 30 degrees so that the closest chief rays (or at least
the paraxial rays) propagate from the image generator 12 toward the first
selectively reflective surface 34 nearly parallel to the second
selectively reflective surface 36 for efficiently filling the prismatic
waveguide 28. Inclination angles of between 25 degrees to 35 degrees are
preferred so that the paraxial rays propagate toward a first encounter
with the first selectively reflective surface 34 within 15 degrees of the
second selectively reflective surface 36.

[0046] Within these preferred bounds, accommodations can be made for
achieving desired ratios of optical path lengths between the first and
second optical paths 26 and 28 in keeping with a focal length "F" of the
selectively reflective focusing optic 30 and the desired magnification of
the design. Generally, the eyebox 14 is located at approximately one
focal length "F" from the selectively reflective focusing optic 30 along
the second optical path 28. The image generator 12, or more specifically,
the object plane 20 within which the image generator 12 generates an
object image, is preferably located at an optical distance along the
first optical path 26 to the selectively reflective focusing optic 30
slightly shorter than the focal length "F", so that the virtual image of
the object image appears to the viewer from within the eyebox 14 at a
distance of approximately three meters.

[0047] Although in the preceding FIGS. 1-3, the object plane 20 of the
image generator 12 is depicted coincident with a surface of the spatial
light modulator 16, the object image produced at the spatial light
modulator 16 or other image generating source can be relayed into the
position shown by relay optics (not shown). In other words, the image
generator 12 can be located remotely, and the object images generated by
the image generator 12 can be relayed to a desired position at a
predetermined optical path length along the first optical path 26 to the
selectively reflective focusing optic 30.

[0048] The devices 10 or 60 illustrated above are preferably supported in
frames (not shown) for positioning the eyeboxes 14 at or near a wearer's
(i.e., viewer's) pupil. Similarly powered devices 10 or 60 can be mounted
for presenting virtual images to both of a viewer's eyes. A filter, such
as a polarization modifier, can be positioned near an entrance to the
see-through pathway 68 to relatively adjust the amount of ambient light
reaching the eyebox 14 with respect to the amount of image bearing light
from the image generator 12 reaching the eyebox 14. Particularly with
respect to the compound display and imaging device 60, the image
generator 12 and camera 70 can be arranged interchangeably. For example,
image-bearing light propagating along the first optical path 26 can be
arranged for transmitting through the first selectively reflective
surface 34 en route to the second selectively reflective surface 36, and
ambient light reflected from the second selectively reflective surface 36
can be reflected by the first selectively reflective surface 34 en route
to the camera 70. These and other modifications, additions, and other
changes will be apparent to those of skill in the art in accordance with
the overall teaching of this invention.